Electrical contact



Patented Aug. 11 195? res ELECTRICAL CONTACT Herbert G. Graves, Jr., Wilkinsburg, Pa.,

to Gibson Electric Company,

assignor Pittsburgh, Pa.,

a corporation of Pennsylvania 13 Claims. (Cl. 200166) .My invention relates to a novel contact for circuit breakers and more particularly to a, novel hard contact comprising silver and a refractory metal and a refractory metal carbide such as any combination of tungsten or molybdenum and molybdenum carbid or tungsten carbide, and a novel process for making the same.

In the operation of circuit breakers, the performance of the contact When the circuit breaker is opened on a fault, materially affects the success of the circuit breaker itself. Circuit breakers are rated in accordance with the amount of current they can carry continually without exceeding specified permissible temperature raises as well as the interrupting rating of the circuit breaker, i. e., the amount of current it can interrupt Without being destroyed. When a circuit breaker contact opens, an arc is formed.

The arc, although of only a very short duration, measurable in fractions of a second, creates a relatively high raise in temperature, which softens the contact and tends to tear metal from the contact. This metal torn from the contact may enter the arc in the form of a metal vapor and condenses on adjoining portions of the circuit breaker, thus reducing its dielectric strength.

In the art of large interrupting capacity circuit breakers two sets of contacts, a main and an arcing, are provided. Contacts comprised of a basic current carrying metal and a refractory material do not weld together and have a high resistance to local surface melting under severe arcing conditions and are therefore desirable for arcing contacts.

However, since contacts of these constituents have a large resistive contact drop and are not good current carrying materials, they cannot be used for the main contacts on a circuit breaker. Constituents for a typical arcing contact are silver-molybdenum or silver-tungsten.

The main contacts are usually comprised of a basic current carrying material and a carbide of a refractory material which provides a contact which can carry large currents, will not oxidize, and is resistive to corrosion.

Examples of thi type of contact are silvertungsten carbideor silver-molybdenum carbide contacts. Since the contacts comprised of these constituents. do not have the heretofore mentioned properties which are desirable for arcing contacts, they cannot be used for that purpose.

.the refractory composition Consequently contacts made of silver and a refractory material are used for arcing contacts and contacts made of silver and a carbide of a refractory material are used for the main contacts of a circuit breaker. However, due to the properties peculiar to each of these contacts, they are not interchangeable in their use. The fact is that in large interrupting capacity circuit breakers the arcing contacts have so much resistance that on the occurrence of a short circuit, these contacts divert the arcing current to the main currents. As heretofore mentioned, the composition of the constituents of the main contacts do not provide them with the property required to withstand arcing contacts and thus these refractive carbide contacts are destroyed on the occurrence of short circuits.

It is one purpose of my invention to provide constituents for a general purpose contact material which can be used as either a main or arcing contact and has properties which are superior to the silver refractory material and silvercarbide of a refractory material contacts heretofore used. The constituents of these superior novel contacts are from the group of (1) A basic current carrying metal, and (2) A refractory metal, and (3) A carbide of a refractory metal.

terial add to the hardness of the contact and asficient quantities of silver.

h Ihhave discovered that I can obtain a relatively in which I use both a refractory composition and a refractory metal carbide. The addition of both and a carbide considerably increases the hardness and thus further reduces the wear of the contact during arcing and, therefore, retaining the dielectric qualities of the circuit breaker, and I have been able to do this without sacrificing the quantity of silver present.

Although it had heretofore been believed that addition of the refractory material were limited by the fact that they restricted the amount of silver that could be possibly mixed with them, I have discovered that by providing them with proper control, I can maintain the desired proportions in a refractory metal and a refractory metal carbide and silver mix. v v I Accordingly, it is an object of my invention to provide a novel contact for circuit breakers composed of a basic current carrying material and a refractory element and a refractory carbide other ob ect of my invention is to provide a novel silver-tungsten-tungs'ten carbide contact for circuit breakers. l 1

Still another object of my invention is to procarbide contact for circuit breakers.

A still further object of my invention is to provide a novel contact of s ilver-tungsteh' 'rriolyb denum carbide, or silver molybdenum-tungsten carbide contact.

Another object of my invention is to produce a novel silver contact having improved circuit breaker performance characteristics.

Still another object of my invention is a method of making electrical contacts which comprises mixing powders of a basic current carrying material, a refractory material and a refractory carbide material.

Another object of my invention is to provide a general purpose contact material for circuit breaker arcing and main contacts.

The proportions and constituents used in carrying out my invention may be varied over a rather wide'range andjcombinations. Although the following is, not to be considered as the only possible composition, it is illustrative of one composition these materials may take:

weer-s of the ,above described ap ro tumbling for several hours in a tumbling mill which also contains clean hardened steel balls. After complete mixing, the powders are then pressed into porous slugs of approximate size and shape. The pressures utilized in making the porous slugs "ma be varied over "a rather wide raiigefs'uch'as' or theorder of '10 to 40 tons er squareinch (t. s'.'i.}'. The greater the-pressures employed, the "greater the density of the slug. Accordingly, the actual pressure used varies in accordance with the desired degree of porosity in the slug, which porosity directly determines the amount of silver which can be impregnated into the slug; I p

Iii 'th'epreferred-procedure, a pressure of 13 t. s'.- i. is used for'pressing the aforementioned illustrative mixture. The resultant porous slug has sceneryof- 9.50 "gins/cc. which represents sewer the ideal or theoretical density of such a miiiture. V I I The slugs are then sintered'for one-half hour indiyhydrogen at 2'000'F. The sintering operation slightly increases the density to 9.89 gins/cc.

Thereafter the porousslug' is impregnated with pure 'silver'bypl'acing a piece of pure s ilver having the "desired weight on top of the porous slug 20 vide a novel silver-molybdenurn molybdenum 4 which is then inserted in a furnace held at a temperature of 2350 F. The atmosphere is reducing in this case and the slug to be impregnated may be left in the furnace for a period of from 3 to '7 minutes depending upon the size of the slug. This treatment results in a product which has a density of 12.8 gms./cc. which is equivalent to 98% of the theoretical density of such an impregnated material. The contact material then displays a Rockwell superficial hardness of T, to 6 0 or Rockwell B hardness 59 to '66. The resultant composition is 37 70 tungsten, 12 /2% tungsten carbide and 50% silver. Three groups of constituents which form the composition of my novel contact are: Group 11 A basic current carrying material such as silver.

Grgtip2; A refractory element such as molybdehuin or t Great 3.- A refractory carbide material such as molybdenum carbide or tungsten carbide.

I 12y mixing one constituent from each group an exact-predetermined composition is obtained with reemage jare first thoroughly mixed by no free graphite. This was not true in the prior art where carbon and tungsten were used, from which tungsten carbideand free graphite was produced. In the prior art method, the amount of tungsten carbide is incomplete and unpredictable; leaving unnecessary free graphite in the commie contact.

As heretofore mentioned, one constituent from each of the above mentioned three groups will form ageneral purpose contact material which can be used for an arcing'or current carrying contact. Although the constituents remain ths'ame, the range of composition by weight are different for the material to be used for arcing contacts and main contacts as illustrated by the following table:

Table I e. Mclybtlcniii'n 25%. V Jim am, refracaa-sm 507 refractory. 20-25%} 0 determined mates roam weight or a'refrac- V tory H position (single refractory element and refractory carol-hens esume the; f contact may housed as an ar ng contact,

whereas the contact is am de of material whichhas 50% by weight'of a refractory composition. I p

The use of a carbide in'a'eanaetinarsthe oxidizingfilm on a refractory element non-66ntinuo sahetiius iiiaks' rhe'cb'riip'csinon tabulated on the 'rightof Tab e desirable for arcing 'conme. How ver, the use or a "carb de unction withareraetor' erhfitilliiiaintainls the current carrying'capaclty of "the inate'rialand hence the composition tabula tedon the. left of Tablet isd sirable for iiiain'coii act's.

compositions of other, "constituents s 15% tungsteh'carbide, 35% tungsten and silver have also roved successful.

However,

since molybdenum is less subjectjto than tungsten and since tungsten "carbide is "indie availablethanniolybdenii 'car bide, the tabulatioh cf the experimenta results or Table if'is limited tb' pcntacts of"silver molybdnum tiihg= Table II Molybdenum (Mo) 18. 9 19. 9 20. 7 21. 8 21. 5 20. 20 22. 3 gsten carbide (W G) 28. 6 30. 14 30. 2 32. 2 32.2 29. .65 22.1 Nickel 9. 5 51 0. 5 Copper l 0.15 .2 9.15 0.2 8 Ver (Ag) 51. 9 49. 3 49. l 45. 8 45. 8 50. 00 55. 4 Hardness Rockwell B scale. 83 87 86 95 98 95 95 Rockwell superficial SOT scale. 72 74 73 80 79 80 80 Average values of I. A. O. S. conductlvity, percent 32. 5 45. 2 y 40. 2 36. 9 45 45 Cross breaking strength lbs/1n} 2 82, 500 73, 000 104, 500 103, 100 100, (100 100,000 I 1 Norm-Constituents are listed by percent by weight.

Although the percentage by weight of silver is smaller than the percentage by volume, these compositions achieve a Rockwell Hardness of B 87 and B 95. Thus the large quantity of silver permits an average I. A. C. S. conductivity range of 32.5 to 45% and still a surprising hardness is achieved.

As noted in Table II small fractional percentages of nickel or copper (or copper and nickel) are added. Since silver does not adhere strongly to carbides, the addition of these materials acts a an adhesive material adhering to itself and the major particles and also makes the contact harder. It should be observed that the addition of nickel and copper do not constitute a limitation, as these materials may be substituted by cobalt, iron, manganese, etc.

The temperature rises obtained during heat tests were lower than those obtained during a similar test on a silver tungsten material having the same volume of silver as the material made in accordance with the present invention. That is, of course, a positive indication that the contact resistance of the new material is lower than that of the straight silver-tungsten or silvermolybdenum materials.

The arcing during short circuit tests on breakers equipped with the contact material made in accordance with the present invention provided for considerably milder displays of sparking than those observed in the standard breakers equipped with silver and refractory content contact material. Similarly, no cotton fires were caused by the new material but such were experienced during short circuit tests on silver-refractory metal materials. This indicates that the standard refractory content material was being eroded, vaporized and blown around within the breaker case to a much greater extent than was the silver-refractory metal-refractory metal carbide material contact. Based upon these test results it would appear that subsequent to short circuit test the circuit breaker dielectric strength would be greater if contacts made in accordance with the present invention were utilized.

This prognosis was confirmed when dielectric test and dielectric breakdown tests were made immediately after the short circuit tests. As had been expected, the dielectric breakdown voltages on the circuit breaker equipped with the silver-tungsten-tungsten carbide mater1al wer e appreciably higher than those for the clrcuit breakers equipped with silver tungsten material.

It is to be understood that the procedure, constituents, composition and all resulting data may vary considerably from the specification without going outside the scope of the invention.

What I claim is:

1. A contact comprising 20-60% of a baslc current carrying material, 2050% of a refractory element and 20-50% of a refractory carbide, the refractory element and the refractory carbide being in the form of a sintered slug, the'pores thereof being substantially filled with said current carrying material.

2. A circuit breaker contact having a final composition comprising 20-60% silver and the balance a refractory material and its carbide, said refractory and said carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

3. A circuit breaker contact having a final composition comprising 20-60% silver and the balance a refractory material and a refractory metal carbide, said refractory and said carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

4. A circuit breaker contact having a final composition comprising 20-60% silver and the balance tungsten and tungsten carbide, said tungsten and tungsten carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

5. A circuit breaker contact comprising 45- 56% silver, 18-23% molybdenum, and 22-33% tungsten carbide, said molybdenum and tungsten carbide being in the form of a sintered slug impregnated with said silver.

6. A circuit breaker contact comprising 50% silver, 20 to 25% molybdenum and 25 to 30% tungsten carbide, said molybdenum and tungsten carbide being in the form of a sintered slug impregnated with said silver.

7. A circuit breaker contact having 35% silver, 3'7 [9 %50% molybdenum and 25-37 tungsten carbide, said molybdenum and tungsten carbide being in the form of a sintered slug impregnated with said silver.

8. A circuit breaker contact having a final composition comprising 36% silver, 48% of a refractory element, and 16% carbide of a refractory element, said refractory and said carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

9. A circuit breaker contact having a final. composition comprising 36% by weight of silver, 48% by Weight of tungsten, and 16% by weight of tungsten carbide, said tungsten and tungsten carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

10. A circuit breaker contact having a fractional percent of copper and/or nickel, having 50% silver and the balance a refractory material and a refractory metal carbide, said refractory material and refractory metal carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

11. A circuit breaker contact having a fractional percent of copper, cobalt, manganese and/or iron, having 50% silver and the balance a refractory material and a refractory carbide, said refractory material and refractory metal 7 carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

12. A circuit breaker contact having a fractional percent of copper and/or nickel, having 25% silver and the balance a refractory material and. a refractory metal carbide, said refractory material and refractory metal carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

13. A circuit breaker contact having a fractional percent of copper, cobalt, manganese and/or iron, having 25% silver, and the balance a refractory material and a refractory carbide, said refractory material and refractory metal carbide being in the form of a sintered slug, the pores thereof being substantially filled with said silver.

HERBERT C. GRAVES, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,145,690 Hensel Jan. 31, 1939 2,154,288 Scholz Apr. 11, 1939 2,160,659 Hensel May 30. 1939 2,299,000 Allen Oct. 13, 1942 2,313,070 Hensel Mar. 9, 1943 

1. A CONTACT COMPRISING 20-60% OF A BASIC CURRENT CARRYING MATERIAL 20-50% OF A REFRACTORY ELEMENT AND 20-50% OF A REFRACTORY CARBIDE, THE REFRACTORY ELEMENT AND THE REFRACTORY CARBIDE BEING IN THE FORM OF A SINTERED SLUG, THE PORES THEREOF BEING SUBSTANTIALLY FILLED WITH SAID CURRENT CARRYING MATERIAL. 